Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a fiber optic pressure sensor, the sensing element may encased within a glass tube or housing comprised substantially of glass. One example of a fiber optic based sensor is that described in U.S. patent application Ser. No. 09/455,867, entitled “Bragg Grating Pressure Sensor,” filed Dec. 6, 1999, which is incorporated herein by reference in its entirety.
The use of fiber optic based devices is widespread in the telecommunications industry, wherein the impervious nature of the glass provides adequate protection given the relatively mild working environments. A relatively recently known use of fiber optic pressure sensors is in an oil well to measure temperature and pressure at various locations along the length of the well bore. The sensors are typically deployed in metal housings in the wellbore and are attached on the outside of the casing. Such sensors may often be subjected to extremely harsh environments, such as temperatures up to 200 degrees C. and pressures up to 20 kpsi. These sensors are exceptionally sensitive and are capable of measuring various parameters, such as temperature and pressure, with extreme accuracy. However, the sensitivity and accuracy of fiber optic sensors creates problems when such sensors are used in a harsh environment. Known problems include poor signal to noise ratios, wavelength drift, wavelength shifts, optical losses, hysteresis and mechanical reliability issues. It is the realization of the these problems and the discovery of the causes that will advance the state of the art in fiber optic based well bore monitoring systems. One such known problem is “creep” of the sensor over time. It has been discovered that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors that measure static or very slowly changing parameters, the long-term stability of the attachment to the structure is extremely important. A major source of such long-term sensor instability is creep, i.e., a change in strain on the sensing element even with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal. Various techniques now exist for attaching the fiber to the structure to minimize creep, such as adhesives, bonds, epoxy, cements and/or solders. In addition, the sensors are subject to fluids containing hydrocarbons, water, and gases that can have deleterious effects on the accuracy of the sensors. For instance, it has been discovered that the performance of wellbore deployed fiber optic sensors is adversely affected by exposure to hydrogen, which causes irreversible loss along the fiber's length. Further, when the fiber optic sensors include Bragg gratings, exposure to hydrogen causes a shift in the index of the grating that severely lessens the accuracy of the sensor. Increased pressure and temperature of the hydrogen increases the rate at which the fiber optic cables and sensors degrade.
It has also been discovered that certain side-hole fiber optic pressure sensors and eccentric core optical fiber sensors experience deleterious effects, such as those described above, when exposed to water at high temperatures and pressures. The adverse effects are presumed to be caused by thin swollen surface layers that lay in close proximity to the sensitive fiber optic core. The observed shifts and changes are presumed to be due to the ingress of water molecules and the subsequent direct expansion of the silica that makes up the fiber itself. In one particular instance, the fibers had a core center-to-surface separation distance of only 10 μm.
However, as discussed hereinbefore, many other problems and errors associated with fiber optic sensors for use in harsh environments still exist. There is a need to discover the sources of these problems and errors and to discover solutions thereto to advance the state of the art in fiber optic sensor use.